1. Field of the Invention
The present invention concerns a method which, by promoting the stabilization permits control of the metalloid concentration of a deposit produced by cold plasma sputtering in reactive vapor phase.
2. Discussion of Related Art
Methods of physical deposit in reactive vapor phase of the type mentioned above, including, for instance, reactive magnetron sputtering in direct current or radiofrequency of a metallic or semiconductive target in the presence of a reactive gas (O.sub.2, N.sub.2, CH.sub.4, B.sub.2 H.sub.6, SH.sub.2, etc.), which provides the metalloid element. The plasmagenic gas responsible for the sputtering of the target is a rare gas, for instance argon. The alloys capable of being synthesized therefore have a base of a metal or of a semiconductor enriched in metalloid (O, N, C, B, S). As to the stoichiometric compounds, they may be oxides, nitrides, carbides, borides or sulfides. These alloys or compounds can be deposited on semi-conductive or insulating metallic substrates.
The sputtering of a metallic or semiconductive target is said to be reactive when it takes place in the presence of a gas which is chemically active with respect to the vapor given off by said target. When, in the presence of an electric discharge which creates a plasma, a reactive gas is introduced into the reactor, a fraction of this gas interacts with the different receiving surfaces (walls of the reactor, substrate, target), any excess fraction being evacuated by pumping apparatus.
This means that the partial pressure P.sub.R of reactive gas is less than that which would be established in the absence of discharge and that the electrical characteristics of the emitting target are affected both by the increasing participation of the reactive type in its sputtering, by the possible modification of the chemical nature of its surface, and by a change in the conditions of excitation and ionization of the plasmagenic gas.
The main difficulty in reactive sputtering is maintaining a stable regime which makes it possible to control the metalloid composition of the deposit, or its stoichiometry in the case of a compound. In fact, experience as well as calculation based on recent models of reactive magnetron sputtering show that this corresponds to a non-linear phenomenon which results in a hysteresis effect appearing on the curves which relate the partial pressure P.sub.R of reactive gas, the concentration C.sub.Me of metalloid in the deposit, the velocity V.sub.D of deposit and the electrical characteristics of the discharge (target current and target voltage for a continuous discharge or auto-polarization voltage for a radiofrequency discharge) to the rate of flow of reactive gas D.sub.R introduced into the reactor.
FIGS. 1, 2 and 3 show the values P.sub.R, C.sub.ME and V.sub.D, respectively, as a function of the rate of flow D.sub.R in the methods of the prior art.
Two sputtering modes can be noted on these curves:
1) For rates of flow of reactive gas less than (D.sub.R).sub.A, the reactive types intercepted by the target are sputtered back before having been able to react with the target in order to form a stable reaction product: This sputtering regime referred to as elementary sputtering regime (ESR), leads to a high deposit velocity and a continuous enrichment of metalloid in the deposit. It is characterized by a low partial pressure of reactive gas, the latter being consumed by reaction on the walls and the substrate with the vapor coming from the target.
2) For rates of flow of reactive gas greater than (D.sub.R).sub.A, the reactive types react very rapidly with the target to form a stable superficial compound having, in general, a low rate of sputtering (oxide, nitride, carbide, boride, sulfide); this sputtering regime, referred to as compound sputtering regime (CSR) results in a sudden increase in the partial pressure P.sub.R of reactive gas due to the saturation of the walls, associated with a large decrease in the deposit velocity. The metalloid concentration of the deposit suffers a discontinuity in order to reach its limit value, which corresponds to that of the richest stoichiometric compound.
The ESR.fwdarw.CSR and CSR.fwdarw.ESR transitions which take place at the critical rates of flow (D.sub.R).sub.A and (D.sub.R).sub.C respectively are unstable and result in a runaway effect which forces the operating point of the reactor to change from A to B or from C to D depending on whether the rate of flow of reactive gas increases or decreases. The AB and CD parts of the curves thus define a region of hysteresis which is wider and more abrupt the greater the difference between the respective rates of sputtering of the target in its elementary state and in its contaminated state.
The existence of a hysteresis makes production with a high velocity of deposition of the richest stoichiometric compound or of deposits the composition of which is within the hatched region of FIG. 2 extremely difficult. As a matter of fact, only a very sophisticated closed-loop control with microprocessor and with very short time of response makes it possible at times to maintain the operating point within the instability regions, for instance by having the rate of flow of reactive gas or the power dissipated on the target controlled by the intensity of an optical emission ray specific to a constituent element of the target. Such a means of control has in the best of cases a response time of a few tenths of a second, which, while suitable for the production of certain compounds, frequently proves much too long, for instance in the case of alumina Al.sub.2 O.sub.3 : as a matter of fact, molecular oxygen can react with the aluminum target by a very rapid chemisorption mechanism since only a few milliseconds are necessary for a chemisorbed monolayer to be formed.
It has therefore been proposed to reduce, or even eliminate, the hysteresis region. It is known that an increase in the pumping speed acts in this direction but, unfortunately, this very rapidly becomes prohibitive in most cases.
The same is true of the artificial decrease of the total area of the receptive walls of the reactor by the use of a sleeve or screens, as well as means which are directed at increasing the reactivity of the metalloid types in the vicinity of the substrate by creating, for instance, a favorable gradient of the reactive gas partial pressure by a suitable selection of its site of introduction into the reactor; these latter methods prove effective only when the reactive gas can naturally interact with the vapor coming from the target, as is true, for instance, of molecular oxygen in its fundamental state.